Julia Armesto

From laser point clouds to surfaces: Statistical nonparametric methods for three-dimensional reconstruction

This paper presents a new method for the reconstruction of three-dimensional objects surveyed with a terrestrial laser scanner. The method is a 2.5D surface modelling technique which is based on the application of statistical nonparametric regression methods for point cloud regularization and mesh smoothing, specifically the kernel-smoothing techniques. The proposed algorithm was tested in a theoretical model-simulations being carried out with the aim of evaluating the ability of the method to filter random noise and oscillations related to the acquisition of data during the fieldwork-and the results were satisfactory. The method was then applied, as a reverse engineering tool, to real-world data in the field of naval construction. A precise solution to the problem of obtaining realistic surfaces and sections of large industrial objects from laser 3D point clouds is provided, which has proved to be efficient in terms of computational time.

High performance grid for the metric calibration of thermographic cameras

The metric calibration of thermographic cameras would make possible the acquisition of geometric data if the principles of photogrammetry are followed. Both the distortion effects introduced in the images by the lens and the perspective effect can be corrected if the calibration parameters are known. This paper presents a calibration grid that allows the automatic metric calibration of thermographic cameras. This calibration grid is made of light and easy-to-find materials to guarantee its portability and handling. The calibration parameters obtained with the presented calibration grid are verified and compared with those obtained with a temperature-based calibration grid through the evaluation of the accuracy and repeatability in the modelling of a standard artefact previously certified.

Error Analysis of Terrestrial Laser Scanning Data by Means of Spherical Statistics and 3D Graphs

This paper presents a complete analysis of the positional errors of terrestrial laser scanning (TLS) data based on spherical statistics and 3D graphs. Spherical statistics are preferred because of the 3D vectorial nature of the spatial error. Error vectors have three metric elements (one module and two angles) that were analyzed by spherical statistics. A study case has been presented and discussed in detail. Errors were calculating using 53 check points (CP) and CP coordinates were measured by a digitizer with submillimetre accuracy. The positional accuracy was analyzed by both the conventional method (modular errors analysis) and the proposed method (angular errors analysis) by 3D graphics and numerical spherical statistics. Two packages in R programming language were performed to obtain graphics automatically. The results indicated that the proposed method is advantageous as it offers a more complete analysis of the positional accuracy, such as angular error component, uniformity of the vector distribution, error isotropy, and error, in addition the modular error component by linear statistics.

Ancient Stone Bridge Surveying by Ground-Penetrating Radar and Numerical Modeling Methods

Bridges are considered necessary engineering structures because they connect separated lands to improve economic and social development. In Spain, many of the bridges in service within the network of transport are masonry arch bridges built in ancient times. In addition to their age, the stability of these remaining bridges is questionable because of the changing loading conditions; therefore, they require periodic assessment of the condition state. Moreover, some of these bridges are considered a part of the cultural heritage of a region, so nondestructive evaluation is required to preserve their historical character. In this work, a medieval stone bridge in the Galician territory of Spain was evaluated using ground-penetrating radar, supported by a detailed geometric survey performed through a terrestrial laser scanner. The results revealed unknown geometrical data and hidden characteristics, including the thickness of ring stones in the interior of the vault, as well as the presence of ancient arches and restorations. To assist in the interpretation, finite-difference time-domain modeling was used, where realistic models were built from the accurate geometry provided. The synthetic data obtained were compared with the field data, which allowed for the identification of unknown structural details.

Trimble GX200 and Riegl LMS-Z390i sensor self-calibration

This paper aims to establish and develop a calibration model for two time-of-flight terrestrial laser scanners (TLS): Trimble GX200 and Riegl LMS-Z390i. In particular, the study focuses on measurement errors and systematic instrumental errors to compile an error model for TLS. An iterative and robust least squares procedure is developed to compute internal calibration parameters together with a TLS data set geo-reference in an external reference system. To this end, a calibration field is designed that performs as an experimental platform that tests the different laser scanner methods. The experimental results show the usefulness and potential of this approach, especially when high-precision measurements are requires.

Robust color texture features based on ranklets and discrete Fourier transform

We present a set of multiscale, multidirectional, rotation-invariant features for color texture characterization. The proposed model is based on the ranklet transform, a technique relying on the calculation of the relative rank of the intensity level of neighboring pixels. Color and texture are merged into a compact descriptor by computing the ranklet transform of each color channel separately and of couples of color channels jointly. Robustness against rotation is based on the use of circularly symmetric neighborhoods together with the discrete Fourier transform. Experimental results demonstrate that the approach shows good robustness and accuracy.

Waveform Analysis of UWB GPR Antennas

Ground Penetrating Radar (GPR) systems fall into the category of ultra-wideband (UWB) devices. Most GPR equipment covers a frequency range between an octave and a decade by using short-time pulses. Each signal recorded by a GPR gathers a temporal log of attenuated and distorted versions of these pulses (due to the effect of the propagation medium) plus possible electromagnetic interferences and noise. In order to make a good interpretation of this data and extract the most possible information during processing, a deep knowledge of the wavelet emitted by the antennas is essential. Moreover, some advanced processing techniques require specific knowledge of this signal to obtain satisfactory results. In this work, we carried out a series of tests in order to determine the source wavelet emitted by a ground-coupled antenna with a 500 MHz central frequency.

Verification artifact for photogrammetric measurement systems

A low-cost mechanical artifact is developed for the metrological verification of photogrammetric measurement systems. It is mainly composed of five delrin spheres and seven cubes manufactured in different sizes. A set of circular targets are fixed on these elements to perform the photogrammetric restitution. The artifact is used in order to compare three photogrammetric systems defined by three different cameras (Canon 5D with 14-mm lens, Nikon D200 with 20-mm lens, and Jai BB500GE with 8-mm lens). Photomodeler Pro and Matlab software are used for the data processing. The precision of the systems is evaluated using the standard deviation of the geometric coordinates calculated from the restitution of the circular targets. The accuracy is calculated using two different procedures: one of them uses the distance between the center of the spheres and the other uses the distance between the faces of the cubes. The comparison between the photogrammetric systems and their associated calibration files reveals that the Jai camera produces the best results in terms of precision and accuracy, while the Canon camera produces the poorest ones. The bad results from the Canon system are primarily related to the low quality of the calibration procedure.

Application of close range photogrammetry to deck measurement in recreational ships.

In this article, we present results that demonstrate the utility of close range photogrammetry in the measurement of decks in recreational craft as an alternate measurement system to the one based on direct acquisition of coordinates. The areas of deck covered with teakwood for aesthetic or security reasons were measured. Both methods were compared in terms of precision of measurements, time consumption, equipment cost, and ease of manipulation and equipment transportation. Based on the results, we conclude that photogrammetry has advantages in almost every aspect with respect to the direct method. Consequently, photogrammetry is suggested as a suitable method for coordinate measurement of decks in recreational ships. However, in some special circumstances, where ships have wide corridors with few obstacles the direct method can be more appropriate than the photogrammetric method.

Metric Potential of a 3D Measurement System Based on Digital Compact Cameras

This paper presents an optical measuring system based on low cost, high resolution digital cameras. Once the cameras are synchronised, the portable and adjustable system can be used to observe living beings, bodies in motion, or deformations of very different sizes. Each of the cameras has been modelled individually and studied with regard to the photogrammetric potential of the system. We have investigated the photogrammetric precision obtained from the crossing of rays, the repeatability of results, and the accuracy of the coordinates obtained. Systematic and random errors are identified in validity assessment of the definition of the precision of the system from crossing of rays or from marking residuals in images. The results have clearly demonstrated the capability of a low-cost multiple-camera system to measure with sub-millimetre precision.